1. Field of the Invention
The present invention relates to a processing system for manufacturing composite cementitious materials as hydraulic blended cements with significantly reduced Portland clinker minerals content. The materials include an increased quantity of supplementary cementitious materials, such as, for example, fly ash, blast furnace slag, fine quartz, granitic quarry fines, and the like, along with highly reactive pozzolans or mineral fillers, such as, for example, fly ash, blast furnace slag, fine quartz, granitic quarry fines, and the like, for direct replacement of a part of the Portland cement in concrete mixtures. Such mixtures are therefore characterized by a significantly improved environmental profile.
2. Description of the Related Art
It is well known that each ton of production of standard Portland cement is accompanied by the release of about one ton of carbon dioxide. About half of that quantity comes from the decarbonation of limestone within the kiln and the other half from energy consumption, primarily within the kiln.
It follows that the only way in which the cement industry can achieve meaningful reductions in carbon dioxide emissions is by reduction of Portland clinker production and an increased use of fillers.
Traditional plants for the production of blended cement include the intergrinding of Portland cement clinker with different types of microfillers, for example, blast furnace slag, fly ash, limestone, and the like, mainly in rotating ball mills. Such methods do not provide more than about 20 to 25% of Portland clinker replacement by fly ash, and approximately 30 to 50% by blast furnace slag, without a significant negative influence on the cement performance, such as a sharp increase of setting time, a very low strength development during the curing period of from 0 to 28 days, and the like. It takes up to 3 times longer, such as 2 to 3 months, to achieve the 28-day strength of traditional Portland cement concretes. At the same time, high volume fly ash (HVFA) cements do have significant benefits in comparison with traditional Portland cements.
The introduction of fly ash or other types of fillers, for example fines from granitic quarries or quartz sand fines, for replacement of Portland cement directly in the concrete mixer in amounts more that 15 to 20% is not efficient, and it could negatively influence the performance of the concrete (reduced strength, etc).
Existing methods of grinding and mechanical activation of such fillers could give some improvement, as described in U.S. Pat. No. 6,630,022, but it appears to be economically ineffective, and it allows replacement of only 20% of Portland cement without experiencing a reduction of the concrete compressive strength.
In International Patent Application No. PCT/SE03/001009 (Publication No. WO 2004/041746 A1) there is described a process for producing a blended cement. The blended cement contains Portland cement that is mixed thoroughly with a microfiller, and possibly a water reducing agent, to a dry cement mixture, along with fine supplementary cementitious materials selected from the materials blast furnace slag, fly ash, quartz, silica, and amorphous silicon dioxide. According to the disclosure in that application, supplementary materials are subjected to a grinding step in a dry state, and the supplementary ground materials are then subjected to grinding together with at least 20% by weight of the total grinding mass of a highly reactive cement mixture in a dry state. A polymer in the form of a powdery water-reducing agent is also added.
A problem in producing such a cement mixture is that in order to obtain the desired result the fineness of the components must be strictly controlled. This is especially true for a plant where the cement mixture is produced in a continuous way.
The present invention is directed to such a continuous production plant.